Active filters have been developed in which the active element utilized is an operational amplifier. These active filters are of very small size and weight compared to passive filter networks and other active filters and eliminate the need for inductors to provide the desired filter transfer functions.
These active filters, even though they have eliminated the need for inductors, still require the use of one or more high quality capacitor filter elements in the circuit having a capacitance generally above about 1,000 picofarads. As the need to make these active filter circuits of still smaller size for modern applications increases, such as in completely integrated circuit designs, the provision of suitable temperature, frequency, voltage, and time stable capacitors which are of small size is continually more difficult to achieve. High quality capacitors having superior stability characteristics are generally quite expensive and large in size whereas physically small capacitors, such as ceramic capacitors, exhibit relatively poor stability characteristics. When these smaller capacitors are used as a part of the filter capacitance, the filter circuit may be highly Q-sensitive to circuit element value changes and may exhibit unstable or severe amplitude peaking or attenuation.
Operational amplifiers, and particularly integrated circuit operational amplifiers, often have a relatively poor frequency response which can by proper design be controlled to an acceptable level. This control may be obtained either by a compensating capacitor, which can be of poor quality and small enough to be incorporated in an integrated circuit chip, or by careful design of the operational amplifier using the parasitic capacitances inherent in the circuit. Typical commercially available integrated circuit operational amplifiers which include compensating capacitors therein are such as the commonly referred to 741-type operational amplifiers. Other types of compensated operational amplifiers include the 536, 107, 5556, 740 and 747. Other operational amplifiers which require an external capacitor compensation may include the 709, 748, 101 and 531 types. These operational amplifier types are available from such manufacturers as the Burr-Brown Research Corporation, Fairchild Semiconductor, Motorola, Texas Instruments, and the like.